Pass known value to PrepareEditor() when deferring preparation.

Differential Revision: https://phabricator.services.mozilla.com/D187981

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1851730
gecko-commit: 72ea7455c255078356b5694d9c94012a3abe5e50
gecko-reviewers: masayuki

editing/crashtests/mozilla-bug-1851730.html

@@ -0,0 +1,11 @@
+<script>
+document.addEventListener("DOMContentLoaded", () => {
+  document.execCommand("selectAll", false)
+  a.wrap = "hard"
+  a.value = "A"
+  b.style.cssFloat = "left"
+})
+</script>
+<textarea id="a" autofocus="autofocus">&quot;</textarea>
+<font id="b">
+<pre>